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Ocean Colour Monitor (OCM)
on Indian Remote Sensing
Satellite IRS-P4

R. R. Navalgund and A. S. Kiran Kumar
Space Applications Centre (ISRO),
Ahmedabad - 380 053
blueline Introduction

While the earlier IRS series of satellites carried cameras (LISS-I, II, III, WiFS & PAN) designed predominantly to meet the needs of land-based applications, IRS-P4 is the first Indian satellite envisaged to meet the data requirements of the oceanographic community. IRS-P4 is planned to be launched by PSLV (Polar Satellite Launch Vehicle)
in March 1999. The payload to be flown on-board IRS-P4 are: (a) OCM (Ocean Colour Monitor) operating in eight narrow spectral bands in the visible / near-infrared region of the electromagnetic
spectrum and with high revisit time (2 days),
and (b) MSMR ( Multi-frequency Scanning Microwave Radiometer) operating in microwave bands 6.6, 10.65, 18 and 21 GHz in dual
polarisation mode. The multifrequency scanning microwave radiometer is envisaged to provide information on physical oceanographic parameters such as sea surface temperature, wind speed and atmospheric water vapour. The IRS-P4 spacecraft will be a polar orbiting satellite in sun synchronous orbit with nominal altitude of 720 km, providing revisit time of 2 days for OCM.

The OCM Instrument

The Ocean Colour Monitor (OCM) instrument designed for the IRS-P4 Satellite programme is significantly different from similar instruments built and launched to date. The main features of the OCM instrument are outlined in Table 1.

Table 1 : Major specifications and features of IRS-P4 : OCM
 1. IGFOV at nominal altitude (m)    360 x 250
 2. Swath (km)    >1420
 3. No. of spectral bands    8
 4. Spectral range (nm)    402-885
 5. Spectral band  
Central wavelength
(bandwidth) in nm
Saturation radiance
(mw cm-2 sr-1 µm-1)
414 (20)
442 (20)
489 (20)
512 (20)
557 (20)
670 (20)
768 (40)
867 (40)
 6. Quantisation bits    12
 7. Camera MTF
(at Nyquist frequency)
 8. Data rate
(Mbits s-1)
 Along Track Steering    +20, 0, 20

The OCM is the first instrument to take advantage of pushbroom technology for achieving higher radiometric performance and higher spatial resolution while maintaining a large swath to provide high revisit time for ocean observations. Unlike SeaWiFS or OCTS the instrument does not have common collecting optics coupled to a scan mechanism for realising the wide swath. The instrument design is an extension of the imaging concept adopted for IRS LISS payloads. The instrument has separate wide angle optics and a linear array CCD detector for each of the eight spectral channels. Individual and separate chains for each of the channels enables optimisation of the performance of one channel without interdependence and hence the need to compromise the performance of other channels. The issue of spectral response variation with large incidence angles is overcome by the choice of telecentric design and use of a spectral selection filter close to the linear array detector.
The pushbroom approach has enabled the use of a 12 bit digitiser to cover the instruments dynamic range. The anticipated radiometric performance is shown in Table 2. The instantaneous geometric field of view of the pixel is 360 meters across track and the sampling interval along track is 250 meters. The instrument is mounted on a mechanism to provide tilt in the along track direction to avoid sun glint.

Table 2 : IRS-P4 OCM - NESR /Dr Values
mw cm-2 sr-1 µm-1
 NE Dr (%)

The spectral bands for IRS-P4 OCM have been selected mindful of the optical properties of phytoplankton pigments (principally chlorophyll-a), inorganic suspended matter and yellow substance, and the requirements of spectral bands for atmospheric correction. The first spectral band centered at 414nm is selected primarily for discriminating Gelbstoffe or yellow substance from viable phytoplankton pigment.
The band at 443nm is close to the absorption maximum of chlorophyll, which is centered at approximately 435nm, but it has been selected because its location minimises interference from a Fraunhoffer absorption line at 435nm. This band is used along with the 557nm band for determining colour boundaries, low chlorophyll concentrations and diffuse attenuation coefficient. The third band, at 489nm, along with a fourth channel at 512nm would allow the use of multi-band spectral curvature algorithms and other second derivative algorithms to be applied to derive chlorophyll concentrations in coastal or Case-II waters. The 512nm band along with a 557nm channel would also be useful in deriving higher chlorophyll concentrations in Case-I waters. The spectral band at 557nm is used as a hinge point for determining chlorophyll concentration and water optical properties such as diffuse attenuation coefficient. The band at 670nm is sensitive to backscattering from suspended matter in coastal waters, and is useful in quantifying suspended matter along with the channel at 557nm. The spectral bands at 768nm and 867nm are used in atmospheric correction procedures.

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The OCM instrument mounted on IRS-P4 will be launched by PSLV and placed in a polar sunsynchronous 720 kilometer altitude orbit. Equatorial crossing is at 12 noon ±20 min, descending node. The satellite has provision for data recording onboard and will also transmit real time data to ground stations in X-band. The ground station at Hyderabad, India will acquire data over the Indian subcontinent and the adjacent Arabian Sea and the Bay of Bengal. IRS-P4 OCM data may also be acquired by other ground stations with suitable augmentation / modification.


Raw Level-1 data will be provided to users. Standard Level-2 products will be generated after applying corrections for radiometric, atmospheric and panaromic corrections. Specific data products can also be generated from OCM data through appropiate pre-processing and applying the essential corrections for atmospheric path radiances. These value-added products will be generated after validation of OCM data on request. Details of the data products are given in Table 3.

Table 3 : Types of data products from IRS-P4 OCM
Type of Products
 Level 1  Browse product  Raw product
 Level 2  Standard product  
 Level 2.1  Raw + Gain & bias  This product is geometrically and radiometrically corrected. Gain and bias values for each CCD element will be provided with data.
 Level 2.2  Radiometrically corrected  Product is geometrically raw. Top of the atmosphere (TOA) data will be provided.
 Level 2.3  Atmospherically corrected  Product is corrected for atmospheric path radiances and panoramic corrections for across track direction.
 Level 3  Special products  The derived products such as chlorophyll, suspended sediment maps.


A comprehensive programme for IRS-P4 OCM data utilisation has been drawn up in India along with a host of users. The IRS-P4 OCM data would be extremely useful for estimation of phytoplankton in oceanic /coastal waters, detection and monitoring of phytoplankton blooms, coastal upwelling, suspended sediment dynamics, location of fronts, identification of water mass boundaries and oil pollution. With additional input from other sensors as well as conventional data, IRS-P4 OCM data will provide detailed information on the coastal region owing to its increased spatial resolution. The information on pigments, in conjuction with sea surface temperature, will greatly assist in identification of potential fishery zones in coastal and oceanic waters. The potential end users of the OCM data products include fisheries management, marine industries, environmental management and studies related to the estimation of primary productivity in the oceanic basins. IRS-P4 OCM, along with other ocean colour sensors such as IRS-P3 MOS, SeaWiFS , MERIS and MODIS will assist the ocean colour community in filling data gaps, and can also be used for the inter-calibration of different ocean colour sensors.


Indian Remote Sensing Satellite IRS-P4 utilisation plan, SAC-RSA/IRS-P4-UP/PP-02/97, Space Applications centre (ISRO), Ahmedabad 380 053, India, May 1997.
Detailed Design and Analysis report on IRS-P4 Ocean colour monitor., SAC/IRS-P4/01/05/98, Space Applications Centre (ISRO), Ahmedabad 380 053, India, May 1998.